Input Validation

Secure File Upload Handling

File uploads cross a security boundary. A file supplied by a user may have a misleading name, unexpected content, excessive size, malicious active content, or a structure designed to exploit the software that processes it. Secure upload handling therefore requires more than checking a filename extension. Treat every uploaded file as untrusted until the application has validated, stored, processed, and served it according to an explicit policy. Start with a narrow upload policy Define what the feature actually needs to accept. An avatar service may need only a small set of image formats, while a document workflow may need PDF files and nothing else.